With the steadily increasing quantities of waste material and the increasingly strict environmental requirements for the incineration of this material, the use of flue-gas cleaning results in the production of increasingly greater quantities of harmful residual products.
U.S. Pat. No. 3,808,989 describes a method of incinerating solid and possibly liquid waste material in a plant, in which a rotary kiln is being used, said kiln being situated downstream from an incinerating section with stepped grates, the solid waste material being partially combusted on the stepped grates and delivered to the rotary kiln and any liquid waste material being added to the waste material being combusted on the stepped grates. The waste material comprises household refuse and partly dewatered sewage sludge and it is for the ability of the plant to combust sewage sludge clumps essential that solid bodies like stone, glass etc. function like balls in a ball mill, breaking up the sewage sludge clumps, whereby a faster and better burning of these is made possible. There is no mention of melting the slag in the rotary kiln which would also make the essential "milling effect" disappear.
In the waste incineration plants known at the present moment, the temperature of combustion is held below 1100.degree. C., e.g. at approximately 875.degree. C., in order to avoid the formation of liquid slag at the side walls of the grate section and in the rotary kiln, this slag having a tendency to solidify at the sides of the furnace and obstruct the exit end of the rotary kiln.
These plants are so adapted and designed that the slag and the ash are discharged as a dry or moistened nonhomogenous mixture respectively in as many as three different discharge assemblies.
Thus, boiler ash is obtained from the boiler assembly, e.g. being of the impact-descaling type, and fly ash from an electrical filter. Further, flue-gas cleaning is performed by adding e.g. lime, after which the flue gas passes through a filter or a flue-gas washing arrangement, in which a residual product is separated.
Further, "small waste" falls through the grate, and such grate screenings are taken to a slag outlet. It is however, necessary that less than 3% of the slag is un-combusted.
These known plants do, of course, suffer from the disadvantages that the salts and heavy metals attached to the slag, the fly ash and the residual products from the flue-gas washing apparatus may be leached out, and some heavy metals evaporate during the process.
For this reason, these products must be deposited in a safe manner, e.g. by being placed in controlled waste-disposal sites, thus producing a percolate of a kind causing considerable damage to the environment, or in salt mines.